The present invention is an apparatus for and method of providing long integration times in an IR detector. Such detectors are useful, for example, in night vision equipment as well for use as cells on forward looking infrared (FLIR) staring focal plane arrays. Such detectors are also useful in detecting wind shear, CO.sub.2 monitoring, respiration monitoring, and visible astronomy. The invention is particularly useful when the detector is a quantum well infrared photodetector.
Recently, semiconductor structures that have dimensions small enough to make quantum mechanical effects important have been described in the literature, e.g., M. Sundaram et al., "New Quantum Structures", Science, Vol. 254, pp. 1326-35 (Nov. 29, 1991). Conventional quantum well infrared photodetectors (QWIPs) respond to thermal radiation with an increase in electrical conductivity that results from internal photoemission of charged carriers from energy states confined in quantum wells. Typical materials used for conventional QWIPs are III-V compound semiconductors such as gallium arsenide and aluminum gallium arsenide. A typical QWIP is a stack of layers in which the layers' materials and widths are carefully selected to achieve a desired distribution of energy states for the device's electrons. See, e.g., B. Levine et al., "New 10 .mu.m Infrared Detector Using Intersubband Absorption in Resonant Tunneling GaAlAs Superlattices", Appl. Phys. Lett., Vol. 50, pp. 1092-1094 (Apr. 20, 1987).
The Levine et al. paper describes an example of the energy states in a conventional, unbiased QWIP. The wells have widths and energy depths that are chosen to provide two confined states, namely, a ground state and an excited state. The energy separation between the ground state and the excited state is set equal to the energy of the photon to be detected by the QWIP, and generally increases as the width of the layers corresponding to the quantum wells decreases. The well layers are doped with electron donor impurities, e.g., silicon, thereby partially filling the lowest energy state with electrons. Barriers having thicknesses of typically one hundred Angstroms (100 .ANG.) separate the quantum wells.
QWIPs have many areas of potential usefulness. Considering the use of a QWIP in a night vision camera, for example, one of the defining figures of merit is the noise equivalent temperature. This figure of merit specifies the minimum observable temperature which the camera can resolve.
The noise equivalent temperature is inversely related to the amount of integration time t.sub.i that is available. This can be shown by an analysis of the detector root mean square noise and the equivalent scene dependent current equation. If photovoltaic detectors such as mercury-cadmium-telluride are used then the noise is found from the expression:
Noise Bandwidth f.sub.n : ##EQU1## Noise Current&lt;i.sub.n &gt; for photovoltaics: ##EQU2##
where q is the electron charge (e.sup.- =1.6.times.10.sup.-19 coulombs) and I.sub.DC is the average current. (For photoconductors, the factor of two under the square root sign becomes 4 g, where g is the photoconductive gain.)
The noise equivalent current depends on the background photon flux. The photon flux is found from Planck's blackbody emittance equation. This classic relationship is: ##EQU3## where: .PHI.(T)=photon flux in photons/sec/cm.sup.2 /Sr
T=temperature in Kelvin PA1 h=6.625.times.10.sup.-34 joule-sec PA1 k=1.38.times.10.sup.-23 joule/Kelvin PA1 c=3.times.10.sup.10 cm/sec PA1 .lambda.=wavelength in cm PA1 .lambda..sub.h =upper or longest wavelength band edge in cm PA1 .lambda..sub.1 =lower or shortest wavelength band edge in cm. PA1 i.sub.p (T) is the photocurrent in amperes PA1 q is the electron charge (1.602.times.10.sup.-19 coulombs) PA1 .OMEGA..sub.r is the solid angle .pi./(2F#).sup.2 =.pi./16 for F#=2 (typical) PA1 A is the effective detector area=16.times.10.sup.-6 cm.sup.2 PA1 .tau..sub.o is the optical transmission efficiency (0.5 typical) PA1 .eta. the conversion efficiency (0.8 typical) electrons/photon. PA1 C.sub.w is the well capacity in farads. PA1 C.sub.w is the well capacitance PA1 C.sub.a is the averaging capacitor.
For .lambda..sub.h and .lambda..sub.1, the band edge is taken as the half power point of the curve, that is, the fifty percent point in responsivity.
The photon flux converts to current in a detector that has a suitable energy gap. The longer wavelengths require smaller energy gaps. The smaller energy gap allows the low energy photons to dislodge the electrons and permit current flow. The relevant flux to current equation for photovoltaics is: EQU i.sub.p (T)=.eta.qA.OMEGA..sub.r .tau..sub.o .PHI.(T) amperes
where:
Again, for photoconductors, the expression on the right is multiplied by a factor of g representing the photoconductive gain.
The flux used here is actually the difference between the flux found at one degree above the background and the flux at the background, divided by one degree times the noise equivalent temperature. Using 300 Kelvin as a reference and setting the electronic noise current equal to the noise equivalent temperature yields the desired relationship. In addition, the direct current is related to the integration time because: ##EQU4## where: V.sub.m is the maximum voltage swing (5 volts typical)
The noise becomes: ##EQU5## Thus, EQU a/t.sub.i =T.sub.ne Kelvin
where T.sub.ne is the noise equivalent temperature and ##EQU6##
The inverse proportionality constant between the integration time and the noise equivalent temperature is thus limited by the well capacity. The well capacity is limited by the IC chip area which is set by the dimensions and spacing of the detector elements. There are thus imposing physical constraints on the range of possibilities for decreasing the noise equivalent temperature. First, the solution cannot use complicated circuitry as there is limited space available to implement the circuit and even less interconnection wiring space available. Secondly, the solution cannot introduce any additional source of noise.
The above analysis concerns itself primarily with photovoltaics. The analysis with respect to photoconductors is substantially the same except that, as noted, photoconductive gain needs to be factored in.
One attempted solution is described in U.S. Pat. Nos. 4,684,812 and 4,686,373 entitled "Switching Circuit for a Detector Array and Infrared Imager" in the name of Tew et al. This technique uses a pair of capacitors. The second capacitor is used to average the ramp and dump cycle of the first capacitor. However, the first capacitor is not reset and starts with the same initial value of charge as the averaging capacitor. Thus, the amount of filtering depends only on the absolute value of the averaging capacitor. Mathematically, the Tew technique is EQU C.sub.eq =C.sub.w +C.sub.a farads
where:
Obviously, the equivalent capacitor depends on the absolute value of the added averaging capacitor.
There remains a need for a method and means by which effective integration times can be increased within the space available on the chip without introducing noise.